The invention relates to a process for treating contaminated water to precipitate metals without increasing the total dissolved solids content. In particular, the invention relates to a process for cleaning water contaminated with hexavalent chrome.
To clean water contaminated with hexavalent chrome, the chrome must be reduced to it's trivalent state prior to precipitation. Hexavalent chrome reduction typically occurs at pH around 3 using an acid, such as sulfuric acid, and a reducing agent (such as sulfur dioxide, ferrous sulfate, sodium metabisulfite, sodium bisulfite, electrolyte iron). The reduction step is followed by a neutralization step which consists of adjusting the pH of the acidic contaminated water to a higher pH to precipitate the trivalent chrome and to meet the discharge permit conditions. The required pH for the chrome precipitation is between 6.5 and 8.2. The most common alkaline reagent for raising the pH is sodium hydroxide. This is due to it's high and immediate solubility in water. These two characteristics result in a reduction in the plant equipment sizes since little residence time is required to achieve the required pH. However, the same two characteristics cause the total dissolved solids (TDS) of the treated water to dramatically increase. The high TDS in the treated water is a problem since there are typically limits on the amount of permitted discharge. To reduce the TDS in the treated water stream, ultrafiltration, microfiltration, and reverse osmosis are used. These processes result in the concentration of the salt (TDS) in a small percentage of the treated stream (around 10%) and produce a filtrate with acceptable levels. The concentrated reject stream remains a problem. Crystallizers or evaporators are also used to further reduce the reject stream volume. The evaporation and filtration units are very expensive, hard to operate, require large space areas for installation, and have a high energy requirement for operation. To eliminate these expensive unit operations, a chemical process which will not cause an increase in the TDS of the treated stream and produce chromium free water is needed.
Thus, there exists a need for a safe, environmentally sound process for the cleaning of metal contaminated water without raising the TDS in the treated water.